Magnetoelectricity (ME) is a characteristic of certain materials and composites, defined as a magnetization induced by an applied electric field and the inverse, i.e., an electric polarization induced by an applied magnetic field. Materials and composites exhibiting ME can be employed for various purposes, including detecting magnetic fields. “Sensitivity,” the ratio of the change in polarization to the change in magnetic field, is one ME performance metric. One measure of sensitivity is the ME field coefficient, or αme, defined as the δE/δH ratio in volts-per centimeter per Oersted (V/cm-Oe).
ME is exhibited by certain single-phase bulk materials, such as Cr2O3 and BiFeO3, and certain single-phase epitaxial films. However, the highest published ME measurement, at room temperature, of Cr2O3 crystals shows a sensitivity of approximately 2.67×10−12 s/m or, equivalently, αme=δE/δH=0.01 V/cm-Oe. See, e.g., V. J. Folen, et al., Phys. Rev. Lett. 6, 607-608 (1961). The sensitivity of such single-phase materials, however, is generally too low to be useful for most applications.
ME effects exhibited by certain multiphase arrangements formed of mechanically coupled magnetostrictive and piezoelectric phases are significantly greater. In fact, the ME sensitivity of such composites can be orders of magnitude higher the sensitivity of known single-phase materials. Therefore, because sensitivity is a critical requirement in various devices and applications employing ME, such composites are almost always utilized.
The general principles by which composites of magnetostrictive and piezoelectric phases exhibit ME effects are straightforward. When the composite is placed in a magnetic field, the magnetostrictive phase undergoes dimensional change. This in turn distorts the piezoelectric phase that is mechanically coupled to the magnetostrictive phase. The piezoelectric phase therefore generates a voltage.
There are many known arrangements of such ME composites. One arrangement is a composite laminate, which is a lamination of layers of magnetostrictive and piezoelectric phases, in a generally alternating sequence. One classification scheme for such composite laminates is the Newnham, which is generally in terms of the respective relative directions of the polarization axis of the piezoelectric layer(s), the magnetization axis of the magnetostrictive layer(s), and the axes in which the layers extend. Known types include the (L-P), the (L-T) and the (L-L), which are discussed in greater below.
Existing composite laminates of magnetostrictive and piezoelectric phases employ a steady-state, or “DC” magnetic field, referenced herein as Hdc to increase the ME sensitivity of the devices. In prior art ME composite laminates high sensitivity requires a high Hdc. High Hdc, though, typically requires high power consumption.
The highest αME attained in the prior art has been termed “giant” ME field coefficients. Examples of such giant ME coefficients have been reported for (L-T) (longitudinally-magnetized transversely-poled) composites of piezoelectric Pb(Zr,Ti)O (PZT) layers laminated with layers of magnetostrictive Tb1-xDyxFe2-y or Terfenol-D (αME(L-T)=2V/cm·Oe), see .S. Dong, J. F. Li and D. Viehland, Appl. Phys. Lett., 83, 2265 (2003), or laminated with Permendur (αME(L-T)=0.8V/cm·Oe), see U. Lestin, et al., Applied Physic A-Materials Science & Processing, 78 (1), 33 (2004); or Fe—Ga (αME(L-T)=0.4V/cm·Oe), see S. Dong, et al., J. Appl. Phys. 97, 10392 (2005); or NiFe2O4 (αME(L-T)=0.45V/cm·Oe), see G. Srinivasab, et al., Phys. Rev. B 64, 214408 (2001).
Further composite laminates having giant ME coefficients αME are disclosed by U.S. Pat. No. 7,023,206 (the '206 patent), issued to inventors of the present invention, which is hereby incorporated by reference in its entirety. FIG. 2 of the '206 patent discloses an example L-P configuration and FIG. 5 of the '206 patent discloses example (L-L) configuration. Magnetic sensors according to the '206 patent's (L-P) configuration have shown ME sensitivities of approximately 55 mV/Oe at a bias of about 500 Oe. Magnetic sensors according to the '206 patent's (L-L) configuration have shown sensitivities of approximately 225 mV/Oe, at a bias in the approximate range of 600-800 Oe.
The αMELL values obtained by the prior art, however, are orders of magnitude lower than the theoretical values, for reasons not known in the prior art.